ISIS claims Raqqa as the capital of its so-called caliphate. The targets in Sunday's airstrikes included a command center, a recruitment center, an ammunition storage base and a training camp for the terror group, said Mickael Soria, press adviser for France's defense minister.

"Clearly, it's a military activity, but it really sends a very strong political message, and it's all for internal consumption within France," said retired Maj. Gen. James "Spider" Marks, a CNN military analyst. "This is very visceral. The types of targets they strike right now really are symbolic. From the French perspective, something has to be done."

But it's difficult to know what's going on inside the ISIS stronghold, said Janine di Giovanni, Newsweek's Middle East editor. And it's also hard, she said, to gauge the best strategy for fighting back.

"I think that it's very complicated, launching airstrikes like this as a retribution, but also as a way of wiping out ISIS," she said. "Because, the other thing is, that you can't wipe out an ideology. You might be able to suppress them militarily, or you might be able to cut off some of their lines, but you can't suppress the key message they're spreading."

On Sunday, the activist collective said that the city appeared to be bracing for an attack even before the French airstrikes began.

ISIS fighters in Raqqa had expected retaliatory airstrikes and evacuated key facilities, including their headquarters, operation and security buildings, a member of Raqqa is Being Slaughtered Silently said.

Streets were empty, the activists said, markets were less crowded than usual and sheikhs in mosques said they expected the city to be struck.

The airstrikes hit several key ISIS facilities, including the city's stadium, activists said, used by ISIS as both its headquarters and a jail. It was not immediately clear what the damage was. So far, according to Raqqa is Being Slaughtered Silently, there have been no reports of civilian casualties.

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The ISIS media wing Amaaq also said the sites hit by airstrikes had been abandoned and that no one had been killed in the airstrikes. CNN has not independently confirmed the groups' reports.

ISIS in Raqqa was previously the target of retaliatory airstrikes in February. Two days after news emerged that the group had burned a captive Jordanian pilot to death, the Middle Eastern nation hit back. At the time, ISIS posted photos of the destruction from the Jordanian airstrikes and the activist Syrian Observatory for Human Rights said 10 militants were killed.